Method for determining capacitance

Abstract

A device and method of determining a capacitance of a device is provided, which in one embodiment includes connecting a first terminal of a capacitor having a known capacitance to the first terminal of the device, applying an AC voltage to the first terminal of the device and the first terminal of the capacitor, measuring a current through the capacitor, measuring a current through the device, determining a first voltage across the device as a function of time, computing a capacitance of the device as a function of time by multiplying the capacitance of the capacitor by the ratio of the current through the device to the current through the capacitor, determining a capacitance of the device as a function of voltage based on the capacitance as a function of time and the first voltage across the device as a function of time, and outputting data of the first capacitance of the device as a function of voltage.

What is claimed is: 
     
       1. A method of determining a capacitance of a device having a first terminal and a second terminal, comprising:
 connecting a first terminal of a capacitor having a known capacitance to the first terminal of the device; 
 applying an AC voltage to the first terminal of the device and the first terminal of the capacitor; 
 measuring a plurality of instantaneous currents through the capacitor while applying the AC voltage; 
 concurrently with said measuring the plurality of currents through the capacitor, measuring a plurality of instantaneous currents through the device; 
 wherein each of the plurality of measured instantaneous currents through the capacitor corresponds in time to one of the plurality of measured instantaneous currents through the device; 
 for each of the plurality of measured instantaneous currents through the capacitor and the corresponding measured instantaneous current through the device, multiplying the capacitance of the capacitor by the ratio of the measured instantaneous current through the device to the measured instantaneous current through the capacitor to provide a capacitance of the device as a function of time; 
 determining a capacitance of the device as a function of voltage based on the computed capacitance as a function of time and a voltage across the device as a function of time; and 
 outputting data of the capacitance of the device as a function of voltage. 
 
     
     
       2. The method according to  claim 1 , wherein said measuring the plurality of instantaneous currents through the capacitor while applying the AC voltage comprises connecting the second terminal of the capacitor to a first channel of an oscilloscope; and
 wherein said measuring the plurality of instantaneous currents through the device comprises connecting the second terminal of the device to a second channel of an oscilloscope. 
 
     
     
       3. The method according to  claim 2 , wherein said connecting the second terminal of the device to the second channel of the oscilloscope, comprises:
 connecting the second terminal of the device to the second channel of the oscilloscope through an amplifier. 
 
     
     
       4. A method of determining a capacitance of a device having a first terminal and a second terminal, comprising:
 connecting a first terminal of a capacitor having a known capacitance to the first terminal of the device; 
 applying an AC voltage to the first terminal of the device and the first terminal of the capacitor; 
 measuring a plurality of instantaneous currents through the capacitor while applying the AC voltage; 
 concurrently with said measuring the plurality of currents through the capacitor, measuring a plurality of instantaneous currents through the device; 
 wherein each of the plurality of measured instantaneous currents through the capacitor corresponds in time to one of the plurality of measured instantaneous currents through the device; 
 for each of the plurality of measured instantaneous currents through the capacitor and the corresponding measured instantaneous current through the device, multiplying the capacitance of the capacitor by the ratio of the measured instantaneous current through the device to the measured instantaneous current through the capacitor to provide a capacitance of the device as a function of time; 
 determining a capacitance of the device as a function of voltage based on the computed capacitance as a function of time and a voltage across the device as a function of time; and 
 outputting data of the capacitance of the device as a function of voltage; 
 wherein applying the AC voltage to the first terminal of the device includes changing an amplitude of the AC voltage; the method further including 
 measuring a response time of the device; and 
 outputting data of the measured response time. 
 
     
     
       5. A method of determining a capacitance of a device having a first terminal and a second terminal, comprising:
 connecting a first terminal of a capacitor having a known capacitance to the first terminal of the device; 
 applying an AC voltage to the first terminal of the device and the first terminal of the capacitor wherein the AC voltage has a period that is less than a characteristic time for the capacitance of the device by factor of at least ten; 
 measuring a plurality of instantaneous currents through the capacitor while applying the AC voltage; 
 concurrently with said measuring the plurality of currents through the capacitor, measuring a plurality of instantaneous currents through the device; 
 wherein each of the plurality of measured instantaneous currents through the capacitor corresponds in time to one of the plurality of measured instantaneous currents through the device; 
 for each of the plurality of measured instantaneous currents through the capacitor and the corresponding measured instantaneous current through the device, multiplying the capacitance of the capacitor by the ratio of the measured instantaneous current through the device to the measured instantaneous current through the capacitor to provide a capacitance of the device as a function of time; 
 determining a capacitance of the device as a function of voltage based on the computed capacitance as a function of time and a voltage across the device as a function of time; and 
 outputting data of the capacitance of the device as a function of voltage.